Increasing evidence suggests that the eradication of minimal residual detectable leukemia cells is necessary for cure. Considerable effort has therefore been made over the past decade to develop sensitive methods to detect these minimal residual leukemic cells in the patient. Polymerase chain reaction (PCR) amplification of non-random chromosome translocations permits sensitive detection of leukemia. However, the majority of children with acute lymphoblastic leukemia (ALL) do not demonstrate such non-random chromosomal translocations, and alternative strategies are necessary to detect minimal residual disease (MRD). In both B and T cell ALL there is usually rearrangement of immunoglobulin (Ig) or T cell receptor (TCR) genes or both, and their clonal progeny bear the identical rearrangement. This unique rearrangement provides a target for amplification and detection of MRD. Moreover, competitive PCR assays can be used to assess quantitatively the tumor burden within the patient. PCR analysis at both the Ig heavy chain locus and the TCR delta locus will be used to amplify the leukemia specific antigen receptor. Sequence analysis of the PCR product will enable us to design junctional specific oligonucleotide probes to detect and quantitate leukemic burden in children with ALL. The basic hypotheses of this proposal are that a rapid reduction in the leukemic burden during induction and intensification predicts for a higher cure rate and that the elimination of detectable leukemia cells is necessary for cure. To this end we propose two specific aims. FIRST: to detect, quantitate and determine the clinical significance of MRD in childhood ALL, to assess the leukemic burden at presentation and to determine whether the magnitude of reduction of leukemic burden during induction therapy as outlined in PROJECT 4 and serially throughout subsequent therapy predicts outcome. In this aim we shall also compare the clinical utility of the detection of MRD in peripheral blood and bone marrow and assess whether the detection of oligoclonal disease or clonal evolution has prognostic significance. SECOND: to assess the clinical significance of MRD detection in children after relapse, to assess leukemic burden using quantitative PCR analysis and to correlate this with clinical outcome, and to determine whether the eradication of MRD is necessary for cure after autologous bone marrow transplantation (ABMT). In this aim we shall determine the clinical significance of the leukemic burden in the patient at the time of and after ABMT and whether PCR detection of leukemia cells in autologous bone marrow or peripheral blood stem cells before and after immunologic purging is associated with poor outcome. PCR analysis will also be performed to assess the impact on MRD of novel treatment strategies outlined in PROJECT 1. Our overall goal is to assess the clinical significance of detection and quantification of MRD to enable us to identify children at high risk of subsequent failure, and just as importantly, to identify those children who may already be cured who could then be spared subsequent toxic therapy. With this approach we should be able to tailor treatment to each individual child based on the risk over time and thereby maximize the therapeutic index.